When a high intensity elastic wave generated by an ultra-sonic power focussing radiator, such as that used for example in extra-corporeal lithotripsy, propagates in a liquid medium, the "negative" half-waves of the wave (i.e. corresponding to pressures less than the mean static pressure) may, in the focal region, generate cavitation phenomena with the creation of gas microbubbles, which hinder the propagation of the wave and may also have undesirable effects on the biological tissues through which they pass.
When the wave has the form of a very brief wave train, this phenomenon results in considerable attenuation of the wave which follows the first negative half-wave, and in this case practically only the first positive half-wave is transmitted which, in general, is not the most important one, whence a loss of efficiency. This problem is stated in the French patent application, filed on the same day by the applicant, which offers a first solution for overcoming this drawback.
It is however possible to work with pulses having negative half-waves, knowing that only the first positive half-wave will be usable, and accepting the loss of efficiency which results therefrom. This solution makes it possible to use simplified generators.
But, in this case, considerable additional disturbances may appear, for certain working conditions, which are superimposed on those already mentioned and further reduce the efficiency of the generator.
These disturbances are due to the accumulation of cavitation bubbles which may occur when working at a high rate, in a confined volume. The cavitation bubbles, which cannot be eliminated quickly enough, form a veritable cushion which very rapidly blocks the whole of the acoustic wave. These conditions are present when firing at a gall stone, generally situated and wedged in a volume of very small size. It can be seen in this case that an increase of the firing rate does not reduce the treatment time, which would be logical, but on the contrary reduces its efficiency. It is therefore not possible to use the possibilities of high rate firing offered by present day extra-corporeal lithotripters.